A decoder for 2-dimensional input devices, such as keyboards or keyboards for the computer system, is always prepared as an integrated circuit (IC) chip. In the computer industry, most commonly used decoder IC's are 40-pad standard decoders. Among the 40 pads, 14 pads (I0-I13) are designated to represent the respective horizontal lines of input keys of a keyboard and 12 pads (P0-P11) are to represent the respective vertical columns of same.
A 2-dimensional input device has a number of input keys arranged in a planar coordinate. Keys belonging to a same horizontal address are electrically connected serially with a horizontal line. Keys belonging to a same vertical address are electrically connected serially with a vertical line. In other words, each key is connected with a horizontal line and a vertical line. These horizontal lines and vertical lines are then connected with their respective corresponding pads (such as I0-I13, P0-P11) of the decoder. When an input key is pressed, variations of voltage will occur in its corresponding horizontal line and vertical line, respectively. This voltage variation, in turn, may be deemed a "1" signal generated by the lines. The signals so generated may be scanned and the coordinate of the pressed input key may be decoded.
The decoder as described above is widely used in keyboards for computer systems personal digital assistants (PDA), telephones, calculators etc. Due to the relatively great number of input keys of the 2-dimensional input device, 40-pad decoder IC's have become a standard decoder for the input devices of these equipments. The cost of such 40-pad decoder IC's has thus become a great burden in the manufacture of these equipments.
It is thus a need in the industry to have a decoder for 2-dimensional input devices with less number of pads. It is also a need in the industry to have a simplified decoder for 2-dimensional input devices so that manufacture costs thereof may be reduced.